Do fish meds evaporate out unlike aquarium salt?

[FriendlyLoach]

Hey everyone,

I know aquarium salt does not evaporate, but what about fish meds, like erythromycin and Paracleanse?  

Thanks!

[Daniel]

How a substance readily something evaporates is related to molecule size. Water molecules are small and evaporate easily. Molecules like erythromycin are quite large and aren't going anywhere anytime soon.

[Daniel]

My answer above didn't feel right, it couldn't just be particle size, because lighter fluid molecules are bigger than water molecules and lighter fluid evaporates more quickly than water. A more dominate predictor of evaporation rates is inter-molecular forces.

I found this paper.

From what I can tell in a quick read, medicines can evaporate to a certain extent. So, that what I get for shooting from the hip! Very interesting topic. Much additional reading is necessary.

 

[Cory]

Without research to make sure I'm right. I believe chemicals precipitate out. I know this is the reason why we can't make fertilizers crazy high strength because at a certain point you're working against yourself. I believe this is also why we can't just put 10x dose of dechlorinator in the water and it'll be good for 10 water changes. It precipitates out over time. From my non chemist brain, chemicals break down over time, mixing with other chemicals can speed up this, cause them to react or not react. Meds can have this same effect, over time they become less potent in the water.

[tekjunkie28]

Both of these answers are correct.... but why do nitrates not evaporate?  That is what I want to know.

[Coronal Mass Ejection Carl]

Nitrate isn't volatile.

[Hobbit]

This is a fun question. Thinking more broadly, one could ask, “How might a substance get out of the water column?”

1. Evaporation
2. Absorption and usage by a living thing (fish, plants, bacteria)
3. Precipitating into solid form
4. Breaking down or reacting with another substance and changing form
5. Bonding to something else and getting stuck (like in a carbon filter)
6. (am I missing anything?)

With fish meds, I always assumed that the fish/bacteria absorb some of them, but also they break down over time. So I think the reason instructions the usually say to add the meds several days in a row is that the meds break down and become ineffective—or at least ineffective for what we need them for—fairly quickly.

Nitrate, on the other hand, is just pretty happy and stable in the water. It doesn’t break down and the water can hold a LOT of it before it precipitates out, and as Carl said, it’s not volatile (meaning it’s not a gas at room temperature). So to remove it we need plants (or bacteria) to use it up, or we need to just change the water.

[Coronal Mass Ejection Carl]

Nitrofuran is said to be unstable in water and also photosensitive.

Kanamycin is apparently stable in water hence the every 3 day dosing.

There's also microbial degradation. For example, bacteria will consume sodium thiosulfate. I'm not sure about other dechlorinating chemicals but I wouldn't be surprised.

You can find studies or data on the persistence of most chemicals in aquatic environments thanks to all the environmental pollution research.

[Squeegee79]

I also feel like meds will break down over time into things that are more inert. (This is just my opinion based on a science degree and a little common sense).

[ChemBob]

Most medications are some form of organic molecules, and are typically very complicated molecules compared to say water and simple oils. They are typically subject to chemical or physical degradation. And small chnage to their structure, either by decomposition or a physical stress causing the molecular orientation to change can change the medications effectiveness. In many cases, only 1 of the many physical arrangements of a molecule is effective as medication because it is the only one physically capable of reaching the site it needs to interact with. If memory serves, many medications also have a large hydrocarbon section (think oil), so they may have a lower solubility in water compared to salts. Depending on a number of factors they may drop out into the substrate over time or rise to the surface and wick away during water changes. I am not a biologist, but this is my memory of biochemistry from 15 years ago. 

Compare this to salts, which are often very simple and stable compounds. They are also ionic, meaning they mix extremely well with water. They are typically very heavy compounds due to high molecular weight, so they are not typically subject to evaporation, instead they remain in the aquatic system, and their concentration can increase of water evaporates away.